I agree that it's important to have a "semi-breathing" world, where there are NPCs, organizations, nations, etc. that have agendas and will be taking actions in the background whether the PCs adapt to them or not. What shouldn't happen (at least in my opinion), is there shouldn't be some final encounter, some end game or plot development that WILL INEVITABLY HAPPEN, no matter what.
My GM-ing took a dramatic leap forward when my focus became about engaging the PCs with what was interesting to the players. To that end, scene frames, situations and NPC motivations have to remain adaptable and fluid. It was about shifting gears and changing directions based on what you see at the table.
I think I understand why you're getting hung up on this point, but the trick to remember is, as a GM you're not authoring "in the moment" all the time. There are absolutely moments where your pre-authored fiction can and should hold true. Sometimes the mace isn't there, and it doesn't matter what the PCs do, or say, or roll on the dice, they ain't gettin' that mace in Location X.
BUT --- and this is the big "but" --- as a GM, you should always be asking yourself, "Is that REALLY the case? Does the game get any better (or worse) if I decide right now, in the moment, that in fact the mace is there? And that the PCs will find it?"
If that's the case, don't even make them roll for it. There's no roll to be made. Assuming the PCs declare that they make any kind of reasonable search of the area to find the mace, they find it. This is classic "Say yes or roll the dice." Well, we just said yes --- the mace is there. Now what happens?
Or if they do roll for it, decide right then and there that they will find SOMETHING, even if the check result says "failure." This is the idea behind "fail forward." Sometimes, the PC's success is guaranteed, but I still make them roll the check to see how long it takes, the degree of success (which can lead to more interesting findings), and whether anything in the scene around them reacts to it.
To me the whole "Climbing Mount Pudding" example is a classic case of process sim run amok. If it's so dang important for the PCs to make it to the top of Mount Pudding, then scene frame it such that the PCs make it to Mount Pudding, but use the climb checks to represent some other variable other than the success of the climb.
Oh sure, if you change your pre-authored fiction, you're probably going to have to change other stuff in the fiction too, and maybe even do a mild ret-con (though in my experience, even if it's a semi-obvious ret-con to the players, most of the time they don't care and just roll with it). But if changing your pre-authored fiction increases the dramatic tension, pushes narrative momentum, and gives the PCs a chance to really dig in further to their character goals.......isn't that BETTER?
(Obviously the hardcore "simulationists" will disagree, but frankly I don't care. In my experience the only time "simulationism" works in the first place is if the players are heavily invested in their PCs with goals, motivations, and back stories . . . in which case, why would I purposefully use heavy process sim / pre-authoring to stunt their ability to engage with what they want?)